EADIATION OF HEAT BY GASES AKD VAPOUES. 
3 
agate plates and other elaborate methods to get rid of the great nuisance of a magnetic 
coil the means of doing so are at hand. Nothing is more easy to be found than 
diamagnetic copper wire. Out of eleven specimens, four of which were furnished by 
Mr. Becker, and seven taken at random from our laboratory, nine were found diamag- 
netic and only two magnetic. 
Perhaps the only defect of those fine instruments with which Du Bois Kaymond con- 
ducts his admirable researches in animal electricity is that above alluded to. The 
needle never comes to zero, but is drawn to it by a minute magnet. This defect may be 
completely removed. By the substitution of clean white silk for green, however large 
the coil may be, the compensator may be dispensed with, and a great augmentation of 
delicacy secured. The instrument will be rendered suitable for quantitative measure- 
ments ; effects which are now beyond the reach of experiment will be rendered manifest ; 
while, the important results hitherto established will be obtained with a fraction of the 
length of wire now in use f. 
§3. 
Our present knowledge of the deportment of liquids and solids, would lead to the 
inference, that if gases and vapours exercised any appreciable absorptive power on 
radiant heat, the absorption would make itself most manifest on heat emanating from 
an obscure source. But an experimental difficulty occurs at the outset in dealing with 
such heat. How must we close the receiver containing the gases through which the 
calorific rays are to be sent \ MellojSTI found that a glass plate one-tenth of an inch in 
thickness intercepted all the rays emanating from a source of the temperature of boiling 
water, and fully 94 per cent, of the rays from a source of 400° Centigrade. Hence a 
tube closed with glass plates would be scarcely more suitable for the purpose now under 
consideration, than if its ends were stopped by plates of metal. 
Bock-salt immediately suggests itself as the proper substance ; but to obtain plates 
of suitable size and transparency was exceedingly difficult. Indeed, had I been less 
efficiently seconded, the obstacles thus arising might have been insuperable. To the 
Trustees of the British Museum I am indebted for the material of one good plate of 
salt ; to Mr. Harlix for another ; while Mr. Lettsom, at the instance of Mr. Darker J, 
brought me a piece of salt from Germany from which two fair plates were taken. To 
Lady Murchison, Sir Emerson Tennant, Sir Philip Egerton, and Mr. Pattison my 
best thanks are also due for their friendly assistance. 
The first experiments were made with a tube of tin polished inside, 4 feet long and 2 '4 
* See Melloni upon this subject, ‘ Thermochrose,’ pp. 31-33. 
t Mr. Beckee, to whose skill and intelligence I have been greatly indebted, furnished me with several 
specimens of wire of the same fineness as that used by Du Bois Eatmond, some covered with green silk 
and others with Avhite. The former were invariably attracted, the latter invariably repelled. In all cases 
the naked wire was repelled. 
j: During the course of the inquiry I have often had occasion to avail myself of the assistance of this 
excellent mechanician. 
B 2 
